1. Field of the Invention
This invention relates to firing arrangements for initiating explosions.
2. Background Information
In conventional commercial demolition equipment, a detonator is used which contains a relatively sensitive primary explosive which is used to detonate a main charge of secondary explosive. When setting up a charge, or a series of charges interconnected by detonating cord, for demolition, it is usual practice to connect the detonator only after all the other components of the arrangement are in place. Thus, the detonation is thereby made possible only at the last moment and the risk of accidents reduced. Alternatively, where a series of charges is used, each is initiated by a separate detonator. Immediately the first charge is connected to its detonator, the remaining ones are made more sensitive to initiation. The use of detonators containing primary explosive means that connected charges are inevitably made sensitive, to the extent that stray electric currents caused by short circuits, or induced by adjacent electrical appliances or wiring, radio transmission or electrical storms, could bring about premature detonation. Also, the detonators, which are of a different hazard class from the main explosive charges, must be stored, transported and carried separately from the main charges, which is inconvenient.
An explosive is classed as a primary or secondary explosive depending on its sensitivity to initiation by temperature, percussion or friction. Primary explosives are highly sensitive and are such that a deflagration-detonation transformation (ddt) takes place when they are present in very small quantities, typically, a few milligrams with minimal confinement. Most primary explosives explode immediately on contact with a flame or spark. If they are capable of burning, then, once ignited, the speed of burning tends to increase rapidly to violent explosion with little confinement. Primary explosives are usually sensitive to detonation as a result of light friction or percussion, exposure to intense electromagnetic radiation (eg. a laser beam), or reaction with disruptive chemicals. Such explosives, which are subject to accidental explosion if carelessly treated, are thus too sensitive to be employed as bulk charges for such applications as blasting, or filling ammunition. Examples of primary explosives are lead and silver azides, and lead trinitroresorcinate, which are employed in detonators and percussion caps.
Secondary explosives are less sensitive than primary explosives and are sufficiently difficult to initiate that the probability of accidental initiation is low enough for them to be used as bulk charges, for example, in blasting and demolition cartridges, bombs and shells. Some examples of secondary explosive are TNT, ammonium nitrate/fuel mixtures, RDX and PETN.